A Multicomponent X-ray Survival Curve for Mouse Lymphosarcoma Cells irradiated in vivo

Abstract

NEARLY all investigations hitherto reported of the survival of animal cell populations exposed to single doses of ionizing radiations have yielded data which have been fitted by curves of the same general shape: a shoulder of varying size (which may be absent) followed by a region of exponential decrease. Only a single exponential component has been found even in those investigations in which survival was studied to fairly low levels (less than 10⁻³), irrespective of whether survival was measured by colony formation in vitro^1–5, or in vivo by tumour induction^6,7. These results have been obtained despite the fact: (a) that a given cell strain may be heterogeneous with respect to innate sensitivity⁸; (b) that cells may undergo marked fluctuations in radiation sensitivity during the division cycle⁹; (c) that inhomogeneities in degree of oxygenation might exist, particularly in a tumour cell population in situ¹⁰. It may be presumed, therefore, that one or more of the following has been pertinent in each study: (1) A particular population may be homogeneous with respect to inherent sensitivity, degree of oxygenation, and presence of protective agents or other modifiers of radiation response. (2) The overall sensitivity of cell populations heterogeneous with respect to radiation response may, under certain conditions, exhibit relatively simple survival kinetics^9,11,12. (3) A less-sensitive cell type may be present in amounts too small to be detected unless many decades of inactivation are effected¹³. (4) The precision of cell survival measurements, particularly those carried out in vivo, is limited.